In fluidized bed incineration processes the combustion occurs in a bed of particulate material (generally sand) heated in a refractory-lined steel vessel. The bed of inert particulate material is supported on a perforated gas distribution plate mounted within the vessel and dividing the vessel into two chambers: the windbox below the plate and the reaction chamber above the plate. The bed is "fluidized" by air passing from the slightly elevated pressure windbox below the plate upwardly through the perforated plate and through the bed at a rate which is a function of the pressure differentials, e.g. bed depth controlled rate.
In a highly desirable mode of operation in large-scale incinerators, when the bed is at ignition temperature the constant feed of combustible waste products to be burned within the bed is at a sufficiently high rate to exclusively fuel the incinerator in autogenic operation (self-sustained burning of the wastes). For the most efficient operation this burning should occur within the bed of granular materials and not in the space of the reactor which is situated above the bed ("freeboard"). In reality however large-scale fluidized bed incineration of waste materials which have high moisture content is fraught with problems due to the requirement that the moisture be vaporized with the resulting cooling effect. Fluidized bed combustion of high moisture feeds typically results in less than complete combustion in the bed, with a substantial proportion of the combustion being "afterburning" or burning which occurs in the freeboard space above the bed. Under such conditions, the temperature in the bed is less than ideal and the temperature in the freeboard is higher than the ideal. Further problems result from the steam which is generated in the combustion of high moisture materials. The steam tends to smother that combustion which does occur within the bed. In exemplary operation, the freeboard should be maintained between 1600.degree.-1800.degree. F. and the fluidized bed should be maintained between 1400.degree.-1600.degree. F. If the temperature in the bed falls much below 1300.degree. F. smoking will begin, indicating a critical condition during which time there is a potential risk of explosion.
In the past this problem of poor combustion in the bed could only be solved by the introduction of an additional fuel (i.e. natural gas or oil) into the bed to supplement the generation of heat and thus significantly raise the bed temperature and consequently lower the temperature in the freeboard space. This need for additional fuel sharply impairs the economics of processes involving fluidized bed incineration of wastes to produce heat derived energy.
Until recently the large-scale commercial fluidized bed incinerators employed most frequently in waste disposal provided for the feeding of the waste material above the fluidized bed through a portal in the side or top of the reactor vessel. In these "top-feed" incinerators, the waste material falls freely through the freeboard or down the sidewalls of the reactor and onto the bed surface. Even with the flow of fluidizing air through the bed, with the resulting circulation and agitation in the bed, this method of feeding still leads to poor distribution of the feed within the bed itself and therefore to less heat generation in the bed. More burning occurs in the upper layer of the bed and in the freeboard space above the bed. It was extremely difficult, if not impossible, to maintain autogenic operation with such systems.
In order to partially alleviate this problem more recent fluidized bed reactor designs have provided for feeding directly into the bed itself. In these reactors the vessel is provided with a chute directed into the bed or a feed portal located lower on the reactor sidewall and adjacent to the bed itself, to permit "in-bed" feeding. The latter method has not been found to solve the poor distribution and poor "in-bed" burning problems in large-scale commercial incinerators. Some smaller incinerator designs have included a stirring apparatus to provide more turbulence and better feed distribution throughout the bed. However, in large-scale operations such stirring has generally not been regarded to be feasible.
Thus, even with the use of in-bed feeding techniques heretofore available, the relatively low extent of burning in the bed and the subsequent greater extent of combustion or afterburning which occurs in the freeboard space above the bed has remained a significant problem, creating a barrier against autogenic operation in the use of fluidized bed incinerators for burning of high moisture wastes.
U.S. Pat. No. 4,036,153 issued to Robin Nauta specifically relates to a "topfeed" incinerator and is claimed to enhance the in-bed distribution of the feed and combustion at or near the top of the bed. The Nauta structure provides the reaction chamber with an auxiliary air source which introduces air at a point directly above the fluidized bed through a pipe which is concentric with a respective downwardly directed feed pipe and is spaced therefrom in surrounding relation to define an annular air passage way. The patent alleges that the air which passes into the reaction chamber through this passage way provides an air shield around the feed pipe and forces material floating on top of the fluidized bed back into the bed. The text of the patent indicates that the relatively dry feed material issues from screw feeders into the downwardly directed feed pipe which ends directly above the bed and the feed falls freely through the pipe and onto the surface of the fluidized bed. The combustion within the bed is supported by the fluidizing air from the windbox below the bed. Column 3, line 19, of the Nauta patent, states that some combustible gasses and fine solids do escape from the bed and that the auxiliary air supports combustion of these escaping materials largely in the region close to, but after they have escaped from the bed. It also states that this combustion takes place in the freeboard but close to the bed. The Nauta patent makes no representations that the Nauta apparatus is capable of autogenic operation, particularly with medium or high moisture level infeeds. It is manifest that the apparatus shown in the Nauta patent is merely a schematic drawing containing absolutely none of the auxiliary accouterments such as auxiliary fuel injection means. The schematic drawings appear to be limited to components germane to the Nauta infeed device. Nonetheless, other than general discussion in the introductory paragraphs there is no specific reference to means for injecting fuel, and raising the temperature of the bed to operating temperature, or means for maintaining ignition temperatures during operation of the bed, even with relatively dry feed material.
Another patent, namely U.S. Pat. No. 3,863,577 which was issued to Andrew Steever et al, described auxiliary air injection means mounted in the reactor wall and disposed in a horizontal plane a short distance above the top strata of the fluidized bed to establish and maintain a combustion zone of high turbulence in the freeboard region which is directly adjacent to and merging with the top strata of the bed. The Steever patent in FIG. 6 and at Col. 8, line 31 shows as one of its alternative embodiments an in-bed feeding system in which sewage sludge is screw-fed directly into the bed. It specifically refers to supply means 62 for the injection of supplemental fuel into the bed (Col. 8, lines 37-40). The Steever's patent also specifically admits, (Col. 8, lines 44-48) that, "These sludges furthermore require the continuous injection of supplemental fuel into the bed in order to maintain the combustion." Nowhere in the Steever patent is there any representation that the Steever's patented apparatus can achieve autogenic operation, i.e. complete elimination of the supplemental fuel.
Both of the above-mentioned patents disclosed auxiliary air sources which provide for air turbulence above and adjacent to the fluidized bed itself. These methods provide for burning in the area directly above the top surface of the bed. Neither patent teaches nor suggests that its respective apparatus can be maintained in autogenic operation.
On the other hand, it is an object of this invention to provide a method and apparatus for the incineration of high moisture (50-70% water) easily friable combustible agglomerates in a fluidized bed on a continuous autogenic basis.
It is an object of this invention to provide for more complete burning of the combustibles within the bed, thus enhancing the heating of the bed itself to the extent that it will sustain autogenic operation and thus reduce the "afterburning" which occurs in the freeboard above the bed.
It is another object to provide for extremely high levels of in-bed combustion in the fluidized bed incinerator, to such an extent that high moisture feeds are continuously dried as well as burned to a great extent within the bed.
It is a further object of this invention to provide autogenic operating conditions, in which the bed is maintained at a temperature of approximately 1500.degree.-1600.degree. F. for the optimum operation without addition of supplemental fuel.
It is a still further object of the present invention to provide an apparatus and method for incinerating high moisture wastes in a fluidized bed system in which the autogenic operating temperature is controllably adjusted upwardly or downwardly by increasing or decreasing the rate of addition of supplemental air into the bed.
Further, it is an object of this invention to provide for combustion of the high moisture materials without the addition of other heat source materials such as oil and natural gas.
These and other objects of this invention are attained by the method and apparatus of the present invention, wherein easily friable combustible agglomerates are fed into the bed in a first high velocity air stream through feed means including a feed tube which is surrounded by a supplemental air tube at least to the point of discharge within the bed. Thus, the injection of the high moisture combustible feed occurs within a second high velocity air stream which surrounds the first stream containing the injected combustibles, at least momentarily after their discharge from the feed tube.
I have discovered that by feeding the combustibles to be burned into the incineration bed in a first high velocity air stream which is surrounded by a second high velocity air stream, in accordance with this invention, a better distribution is provided and thus more rapid and complete burning of the combustibles occurs in an autogenic operation, even with high moisture feed, e.g. 50-80% water.